Light Cycles: Should you leave the lights on?

The short answer is that people have had success growing under constant-light conditions as well as using 18/6 hour light cycles to imitate daylight. Why leave the lights on? Proponents say leaving the lights allows more photosynthesis, which means larger and faster plant growth. The counter-argument is that plants have adapted in the wild to grow in circadian cycles of light and darkness.

Plant species are an important consideration: some plants, like tomatoes, do not do well under constant light. In fact, some plants can get “high-light” injury from too much exposure, leading to yellowing, poor growth, and other symptoms.

The type of light used is also a factor. Studies done using variations of light intensity and temperature to simulate a circadian cycle and have shown success, but the research is still in its early stages. But for those of us with a single light setup, there aren’t many options besides “on” or “off.” Hardy plants would likely not suffer from being under constant light, and will offer increased yield in those conditions.

But remember: size isn’t everything. A larger plant does not necessarily offer more quality than a smaller specimen grown with more patience and in conditions closer to what you’d find in the wild. For instance, recent studies have shown that increased CO2 can lead to increased plant growth but with less of the trace micronutrients we normally depend on.

However you decide to light up your plants, it may be worth it to switch off the lights once in a while—you might be surprised by what you get.